Support JSON.mapping better

[waterlink]

Currently, JSON.mapping overrides .new({ ... => ... }) constructor with its own.

Workaround for now is to do:

class Post < ActiveRecord::Model
  # ...

  def initialize(h : Hash)
    super(h)
  end

  JSON.mapping({ ... })
end

[waterlink]

This actually will still not work. Since JSON.mapping expects mapped fields to be in @<field_name>, but they are in @typed_fields[<field type>][<field name>] at the moment.